1. Field of the Invention
The present invention generally relates to an image forming apparatus utilizing an electrophotographic process and, more particularly, to the image forming apparatus wherein an electro-optical material such as PLZT (a kind of solid-solution ceramic material expressed by a chemical formula, (Pb.sub.0.921 La.sub.0.079)(Zr.sub.0.70 Ti.sub.0.30).sub.0.98 O.sub.3) is employed for a light shutter and wherein a drive voltage proportional to information to be reproduced is selectively applied to a plurality of electro-optical materials to vary the amount of light passing through the light shutter so that the light passing through the light shutter can be utilized to form a recording by means of the electrophotographic process.
2. Description of An Prior Art
The image forming apparatus utilizing the light shutter is well known in the art. In this image forming apparatus, the electro-optical materials are exposed to rays of light at all times and, when a drive voltage is applied to some of the electro-optical materials, light modulating portions of some of the electro-optical materials permit the passage of recording light therethrough. The drive voltage is generally of a value required to cause the electro-optical materials to form an electric field in a predetermined direction. It has been found that a repeated application of the drive voltage to the electro-optical materials for a substantial period results in an internal polarization, that is, light-induced fatigue, occurring in some of the electro-optical materials, which in turn results in a change in drive voltage of a value required to maximize the light transmission through the light shutter, that is, a so-called half-wavelength voltage.
In order to recovery the electro-optical materials from the light-induced fatigue thereby to avoid any possible change in the half-wavelength voltage, some of the inventors of the present invention have suggested an image forming method wherein, during a non-recording period in which no recording is carried out, a recovery voltage intended to recover the electro-optical materials from the light-induced fatigue is applied to the electro-optical materials so as to form an electric field in a direction opposite to that formed by the application of the drive voltage during a recording period. This suggested method is disclosed in U.S. Pat. No. 4,902,111 issued Feb. 20, 1991.
On the other hand, in the image forming apparatus utilizing an electrophotographic process, a recording of an image on a recording medium such as, for example, paper is carried out by radiating imagewise rays of light to a photoreceptor surface to form an electrostatic latent image and then applying toner material to the electrostatic latent image to form a toner image which is subsequently transferred onto and fixed on the recording medium. This electrophotographic process may possibly be classified into two types depending on the manner by which the electrostatic latent image is developed into the toner image. One type may be referred to as a positive-to-positive system wherein the portion of the electrostatic charge built up on the photoreceptor surface which has been exposed to the imagewise rays of light is depleted to form a positive electrostatic latent image on the photoreceptor drum and, during a developing step, toner material charged to a polarity opposite to that of the positive electrostatic latent image is applied to the remaining portion of the electrostatic charge, that is, the positive electrostatic latent image, thereby to form the toner image. The other may be referred to as a negative-to-positive system wherein a portion of the electrostatic charge built up on the photoreceptor surface which has been exposed to the imagewise rays of light is depleted to form a negative electrostatic latent image on the photoreceptor drum and, during a developing step, toner material charged to the same polarity as that of the negative electrostatic latent image is applied to that portion of the electrostatic charge, that is, the negative electrostatic latent image, thereby to form the toner image.
According to the previously discussed image forming method, while the recovery voltage, that is, the voltage used to recover the electro-optical materials from the light-induced fatigue, is applied to the electro-optical material during the non-recording period, it has been found that the application of the recovery voltage tends to allow the rays of light to pass through a portion of the light shutter where the recovery voltage has been applied. Because of this, where the prior art light shutter is employed in the electrophotographic image forming apparatus utilizing the negative-to-positive developing system, the application of the recovery voltage takes place during the non-recording period and does, therefore, not affect the recording.
However, a relatively large quantity of toner material tends to be deposited during the developing step on a portion of the photoreceptor surface which is exposed to rays of light having passed through a light modulating portion as a result of the application of the recovery voltage. The toner material so deposited on that portion of the photoreceptor surface does not participate in the recording and is subsequently removed therefrom by a cleaning means without being transferred onto the recording medium, thus posing a problem associated with a waste of toner material.
Also, repeated deposition and removal of the toner material in relation to the photoreceptor surface tends to allow toner material to scatter and, therefore, not only the inside of the image forming apparatus, but also some recording mediums tend to become dirty.